Construction machines using such chisel devices and chisels are, in particular, ground milling machines such as road millers, recyclers, stabilizers or surface miners, often used in road construction or path construction or for the surface extraction of natural resources. They comprise a machine frame, an operator's platform, multiple crawler tracks, and a driving engine, usually a diesel engine. The key element of a ground milling machine is a milling drum, usually mounted for rotation within a milling drum box. As a rule, such a milling drum is characterized by a supporting tube in the shape of a hollow cylinder, on the cylindrical outer surface of which a plurality of milling tools is disposed. These milling tools are often designed as chisel devices, each comprising at least one chisel holder connected to the supporting tube and one milling chisel mounted in said chisel holder. During operation of the ground milling machine, the milling tools are driven into the ground by the rotation of the milling drum, thus milling off the ground material to the desired milling depth. The milled material is subsequently conveyed, for example, via a discharge conveyor, either in, or contrary to, the direction of advance, onto a transport vehicle and dispatched.
During operation, the milling chisels, in particular, are naturally subjected to a high degree of stress and, thus, to a comparatively rapid wear rate. Regular replacement is therefore necessary. To minimize the resulting costs, there is need for milling chisels having a maximum lifespan which are at the same time of a highly simple and affordable design.
A generic chisel device comprises a chisel holder and a chisel, in particular, a round-shaft chisel. The chisel holder is mounted on, for example, welded to, the cylindrical outer surface of the milling drum and the milling chisel is inserted into a mounting orifice of the chisel holder and held in position therein in such a manner that, when worn, it can be dismantled by an operator as quickly and straightforwardly as possible for replacement by a new milling chisel. Apart from single-piece variants, the chisel holder can also comprise multiple, in particular, two, sub-units, for example, one basic holder and one quick-change tool holder. In this case the basic holder is mounted on the milling drum. The quick-change tool holder is detachably fixed to the basic holder, the milling chisel being held by the quick-change tool holder. In that arrangement, both the milling chisel and the quick-change tool holder can be quickly and easily replaced in the event of wear.
A milling chisel typically comprises a basic body of, in particular, uniform material, for example, steel, and comprising a shaft and a head. The head of the milling chisel usually runs to a tip. The tip may consist of a different material, for example, carbide, and can be braze-welded to the basic body. The tip is situated, at least in part, in front of the basic body as regarded in the direction of advance of the tool. During the operation of the ground milling machine, the milling chisel engages the ground by way of the tip and mills the ground. The direction of advance of the tool is thus the direction in which the milling chisel engages the ground to be milled and is driven into the ground. To simplify, it may be stated that the direction of advance of the tool extends parallel to the longitudinal axis of the milling chisel and runs from the shaft towards the tip. In the case of a round-shaft chisel, the transverse axis coincides with the symmetry axis of the round-shaft chisel. The milled material removed from the ground glides past the chisel head and parts of the chisel holder. For the purpose of causing the wear on the chisel to be as even as possible, the chisels are often designed in such a way that they revolve about their own axis during operation. According to its function, the chisel can be divided into two regions. The tool region is that part of the chisel that protrudes from the chisel holder and comprises the head, the tip, and further devices on the milling chisel. This part of the milling chisel is in direct contact with the milled material, resulting in a great amount of stress and wear on the material, particularly in this region. In other words, the tool region defines that region of the milling chisel that protrudes from the chisel holder during operation or, more particularly, in the position in which it is inserted into the chisel holder. The second region is the shaft region or, more particularly, the holder region, substantially comprising the milling chisel shaft and surrounded by the chisel holder during operation and covered externally by the holder.
Generic chisels are disclosed, for example, in DE 31 12 459 A1. Described therein is a chisel comprising a supporting body made of steel and a chisel jacket body including a tip made of ceramic material. The ceramic material is provided to cause less sparking during the milling operation, as may be particularly relevant in mining situations in which explosive dust/air mixtures or gases are present. In the case of generic ground millers, ceramic chisels have not become established due to the fact that their manufacturing costs are significantly higher than those of carbides. In addition, milling chisels having carbide tips are disclosed, for example, in DE 40 39 217 A1. In addition to the tip, a wear-proof layer is applied to the chisel head to prevent fracture of the head. Neither does this solution, however, result in a satisfactory lifetime of the milling chisels.
It has been found that many chisels suffer wear due not to wear of the carbide tip but to wear of the steel chisel head, necessitating replacement thereof. The milled material moving past the head results in a continuous loss of material at the chisel head to possibly such an extent that the carbide tip, although still fully functional, loses its hold on the chisel head. In other words, the holding material for the tip is eroded during operation. This means that the comparatively expensive carbide tip cannot be used over its entire life and that, instead, an early replacement of the entire chisel becomes necessary despite the fact that the lifespan of the tip has by no means been exhausted. Furthermore, at the same time damage to the chisel holder may often occur due to such breakage, resulting in the chisel holder also having to be replaced, which increases the costs still further. Furthermore, the prior art tips cannot be glued to the milling chisels, due to the occurring shearing loads, but must be soldered to the relevant supporting parts of the milling chisel. Soldering leads to an influx of heat into the material, which changes the structure of the basic body. For the purpose of obtaining a stable end product, such changes must be compensated for after soldering by way of further heat treatment, which further complicates the production and increases the production costs.
It is thus an object of the present invention to provide a chisel device for a ground milling machine and a milling chisel for said chisel device that can be produced at low cost and that are characterized by an increased lifetime. In particular, it is desired to prevent break-off of a still functional carbide tip in the chisel head region.